In general, this kind of a conductor composition comprises conductive particles with electrical conductivity and a solvent. Specifically speaking, for example, an electroconductive adhesive is known, wherein Ag fillers with particle diameters at the level of a micron or a sub-micron (in the order of 1 μm to 10 μm) was dispersed in a solvent and a binder resin such as an epoxy resin, etc. (refer to e.g. Japanese unexamined patent publication No. 2000-319622).
Further, as this kind of the conductor composition, a paste composition has been suggested, wherein fine metal particles with a mean particle diameter from 1 nm to 100 nm were covered on the surface with an organic compound which was able to coordinate with a metal atom contained in fine metal particles, and were stably dispersed in a liquid (refer to e.g. Japanese unexamined patent publication No. 2002-299833).
Such an electroconductive adhesive was used in order to form an electrical connect between a surface mounting component and the mounting substrate, by being placed between them, and to construct the mounting structure, when the surface mounting component was mounted on an electrode formed on one side of the mounting substrate.
This kind of conductor composition has been used also to form surface wiring, inner-layer wiring and a via conductor on a mounting substrate such as a print substrate, a ceramic substrate etc.
In the above-mentioned general conductor composition such as e.g. the electroconductive adhesive containing the Ag fillers, a connection was completed by being placed on the substrate, with steps of heating after being applied on a substrate such as an electrode, etc., evaporating a solvent and curing a binder resin.
Herein, a curing treatment for the electroconductive adhesive was, in general, carried out under a low temperature (e.g. about 150° C.) necessary to remove the solvent, or cure the binder resin.
In a case where such a process under the low temperature was achieved, an electrically conducting structure at the treated electroconductive adhesive was due to connections between Ag filler particles (conductive particles). Due to the connections, an electric resistance at the electroconductive adhesive dominantly depended on the contact resistance between these Ag filler particles.
Therefore, the conductivity of the electroconductive adhesive was approximately two orders of magnitude lower than the inherent conductivity of Ag, i.e. the conductivity at the same level as in an Ag bulk. Further, for the electrically conducting structure between the electroconductive adhesive and the substrate, there has been a problem of low reliability of the connection, due to electrical conduction by contact between the substrate and the Ag fillers.
For these problems, when the electroconductive adhesive (the conductor composition) is processed, it is deemed to be better to treat the adhesive at a temperature high enough to melt the conductive particles of the Ag fillers. It seems that, by the process, the electrically conducting structure at an inner part of the electroconductive adhesive and between the electroconductive adhesive and the substrate becomes a structure for an electrical conduct between the Ag fillers mutually melted together, and conductivity at the same level as Ag bulk can be obtained in the electroconductive adhesive.
However, in order to achieve such a melting situation, it is necessary to increase the temperature for treating the electroconductive adhesive to, for example, close to the melting point of Ag (about 800° C.). Considering the heat resistances of an electronic component, which forms the substrate material, and the substrate, such a process is not practical.
Against that, as described in Japanese unexamined patent publication No. 2002-299833, a conductor composition composed of only fillers (conductive particles) with a particle size of the order of nanometer has been suggested. The composition substantially had a structure wherein only the fillers with a smaller size, of the order of nanometer, than conventional particles, with a particle size of the order of micron, were dispersed in a solvent and a binder resin.
Accordingly, it is expected to have fillers melted together by making the fillers small in the order of nanometer, and decreasing the melting temperature of the fillers themselves, even if they were treated at a low temperature (e.g. about 150° C.) like the low temperature for processing the conductor composition mentioned above.
However, in a case of the conductor composition composed of only nanometer order fillers, their handling is difficult due to the small size of the filler particles, and their cost is higher in comparison with the conventional micron order filler particles.
As the filler particle size of the order of nanometer is much smaller than the conventional filler particles, the small filler's dispersing property, into the solvent and the binder resin, is poor. As a result, it has been difficult to obtain a practical conductivity, owing to the filler particles being unevenly distributed in the conductor composition, and the filler particles do not evenly melt together.
Therefore, considering the above-mentioned problems, the present invention was achieved to provide a conductor composition which can easily secure conductivity at the same level as Ag bulk with a low temperature treatment, a mounting substrate utilizing such a conductor composition, and a mounting structure utilizing such a conductor composition.